1. Field of the Invention
The present invention relates in general to systems and methods for controlling operation of an automotive internal combustion engine based on an information signal issued from an oxygen sensor installed in an exhaust system of the engine, and more particularly to systems and methods for estimating the temperature of the oxygen sensor to measure the current activating degree of the oxygen sensor. More specifically, the present invention is concerned with the estimating systems and methods of a type which estimates the temperature of the oxygen sensor based on an operating condition of the engine and a driving condition of an associated motor vehicle.
2. Description of the Prior Art
Hitherto, in motor vehicles powered by an internal combustion engine, there have been widely employed a so-called air/fuel ratio feed-back control system which controls operation of the engine based on an information signal issued from an oxygen sensor installed in an exhaust system of the engine. In fact, as the oxygen concentration in the exhaust gas has a close relation with an air-fuel ratio of the mixture fed to the engine, the control of engine operation is so made as to bring a current air/fuel ratio to a target air/fuel ratio with reference to the information signals issued from the oxygen sensor.
As is known, since the intrinsic function of the oxygen sensor is exhibited only when the sensor (more specifically, oxygen sensing element thereof) is heated up to its activating temperature, the air/fuel ratio feed-back control is started once the sensing element of the oxygen sensor is heated up to such activating temperature.
Hitherto, in the air/fuel ratio feed-back control system, for the need of diagnosing the responsibility of the oxygen sensor, the temperature of the sensing element of the oxygen sensor has been estimated based on the temperature of engine cooling water and the time elapsed from engine start. One conventional technique for estimating the oxygen sensor temperature is described in Japanese Patent First Provisional Publication 7- 269401.
However, due to inherent construction, the above-mentioned conventional temperature estimating technique tends to produce a non-negligible error in estimating the temperature of the oxygen sensing element under a certain operating condition of the engine and a certain environment. Such error causes the air/fuel ratio feed-back control system to fail to exhibit its intrinsic function. That is, such error obstructs the control system from not only feeding the engine with a mixture of a desired air/fuel ratio but also carrying out a proper diagnosis on the responsibility of the oxygen sensor.